1. Field of the Invention
The present invention relates to the epitaxial growth of III-V compounds. The present invention also relates to the growth of epitaxial films on a non-native substrate by a sequential deposition process. The invention further relates to the growth of epitaxial films by a process in which the growth system is cleaned repeatedly during epitaxial growth of a film.
2. Background of the Related Art
Hydride vapor-phase epitaxy (HVPE) is an important technique for the epitaxial growth of various semiconductors, such as gallium nitride (GaN). Gallium nitride is emerging as an important technological material. For example, GaN is currently used in the manufacture of blue light emitting diodes, semiconductor lasers, and other opto-electronic devices. The background of the related art will be discussed using deposition of GaN epitaxial layers by HVPE as an example.
In HVPE systems, growth of GaN proceeds due to the high temperature, vapor-phase reaction between gallium chloride (GaCl) and ammonia (NH.sub.3). The two gases are directed towards a heated substrate where they meet and react to produce solid GaN on the substrate surface. There are, however, certain difficulties associated with HVPE. For example, the reagent gases can react before reaching the substrate, leading to premature deposition of GaN, i.e. GaN deposition on non-target surfaces. The substrate typically rests on a susceptor which is arranged at an angle with respect to the direction of gas flow (e.g., refer to
FIG. 2). The entire susceptor, and not just the substrate, is maintained at the elevated temperatures necessary for deposition to occur. Thus, growth of solid GaN can occur on the susceptor upstream from the substrate. Such upstream GaN deposits have negative consequences for crystal growth on the substrate. Firstly, deposits of solid GaN tend to obstruct proper flow of reagent gases towards the substrate. Also when unwanted GaN deposits accumulate beyond a certain thickness, they tend to merge with the epitaxial layer. Merger of unwanted GaN deposits with the epitaxial layer degrades the uniformity of the GaN epitaxial layer and leads to an inferior product. Further, unwanted GaN deposits that occur upstream can result in particles depositing on the substrate during the epitaxial layer growth, leading to layers with rough surfaces and poor crystalline quality.
According to prior art methods for forming thick layers of GaN on a substrate, excessive deposition on the susceptor stage can reach problematic levels during growth of the GaN film. However, by repeatedly interrupting the growth cycle and forming epitaxial layers sequentially on the substrate, unwanted deposits may be removed from the susceptor and other system components, in situ, by removing the sample and passing an etchant gas through the reactor. While sequential epitaxial growth is more time-consuming, it provides an epitaxial layer having improved surface morphology.
Furthermore, the repeated interruption of the growth cycle (which is a feature of such sequential epitaxial growth) has the added advantage of solving a second drawback to epitaxial deposition of GaN by HVPE. It is currently impossible to fabricate bulk GaN crystals of usable size as substrates in semiconductor manufacturing. Thus, GaN films are made by deposition on a non-native substrate material. However, a thermal mismatch exists between the non-native substrate and the GaN layer. After epitaxial growth of a layer of GaN is complete, the sample must be removed from the HVPE reactor. This involves cooling the sample from the growth temperature to ambient temperature. During cooling, thermal mismatch between the substrate and the epitaxial layer causes stress to develop in the sample. In the case of a relatively thick GaN layer, the entire sample often shatters into small, unusable pieces. By sequentially forming a number of relatively thin epitaxial layers and allowing the sample to cool between deposition of each layer, stress due to thermal mismatch is dissipated periodically, thereby preventing shattering of the entire sample. Thus, sequential epitaxial growth according to the invention also solves the prior art problem of disintegration of epitaxial films grown on non-native substrates.
U.S. Pat. No. 4,632,712, and its continuation U.S. Pat. No. 5,091,333, both to Fan et al. disclose a method for reducing dislocations in semiconductors, in which growth of a semiconductor is interrupted and thermal cycling is used to trap dislocations in the initial stages of epitaxial growth, thereby reducing dislocation densities in the active top layer. Dislocations may be trapped by cooling or by heating the deposited portion, or by a combination of heating and cooling.
U.S. Pat. No. 5,424,243 to Takasaki discloses a method of forming a compound semiconductor structure in which a stacked amorphous layer of a group III-V compound is subjected to a cyclic annealing process. The annealing process causes crystal defects existing in a GaAs crystal layer to move to, and be absorbed by, an amorphous GaAs layer.
U.S. Pat. No. 5,686,738 to Moustakas discloses a method of preparing GaN single crystal films by molecular beam epitaxy. Growth occurs via a two step process using a low temperature nucleation step (at 100-400.degree. C.) and a high temperature growth step (at 600-900.degree. C.).
U.S. Pat. No. 4,246,050 to Moon discloses liquid phase epitaxy for growing a lattice matched layer of GaAsSb on GaAs substrates through a grading layer of AlGaAsSb. Dislocation defects are more evenly distributed over the surface of the growing layer by a step cooling procedure.